A prior art accelerometer with high performance potential is described in U.S. Pat. No. 3,702,073. The accelerometer comprises three primary components, a reed, and upper and lower stators or magnetic circuits between which the reed is supported. The reed includes a movable paddle that is suspended via flexures to an outer annular support ring, such that the paddle can pivot with respect to the support ring. The paddle, flexures and support ring are commonly provided as a unitary structure composed of fused quartz.
Both upper and lower surfaces of the paddle include capacitor plates and force rebalancing coils. Each force rebalancing coil is positioned on the paddle such that the central axis of the coil is normal to the top and bottom surfaces of the paddle, and parallel to the sensing axis of the accelerometer. A plurality of mounting pads are formed at spaced-apart positions around the upper and lower surfaces of the annular support ring. These mounting pads mate with inwardly facing surfaces of the upper and lower stators when the accelerometer is assembled.
Each stator is generally cylindrical, and has a bore provided in its inwardly facing surface. Contained within the bore is a permanent magnet. The bore and permanent magnet are configured such that an associated one of the force balancing coils mounted on the paddle fits within the bore, with the permanent magnet being positioned within the cylindrical core of the coil. Current flowing through the coil therefore produces a magnetic field that interacts with the permanent magnet to produce a force on the paddle. Also provided on the inwardly facing surfaces of the stators are capacitor plates configured to form capacitors with the capacitor plates on the top and bottom surface of the paddle. Thus movement of the paddle with respect to the upper and lower stators results in a differential capacitance change.
In operation, the accelerometer is affixed to an object whose acceleration is to be measured. Acceleration of the object along the sensing axis results in pendulous, rotational displacement of the paddle with respect to the support ring and the stators. The resulting differential capacitance change caused by this displacement is sensed by a feedback circuit. In response, the feedback circuit produces a current that, when applied to the force balancing coils, tends to return the paddle to its neutral position. The magnitude of the current required to maintain the paddle in its neutral position provides a measure of the acceleration along the sensing axis.
In an accelerometer of the type described above, the reed or acceleration sensitive structure is clamped between the upper and lower stators. The "preload" force required to clamp the reed between the stators is commonly provided by means of a band, commonly known as a belly band, that encircles the reed and the adjacent stator portions, and that is bonded to the stators. The advantage of such a preload system is that it exerts the preload force about the peripheries of the stators, adjacent to the mounting pads, thereby minimizing the introduction of uneven preload forces. However, the disadvantage of such a preload technique is that if subsequent testing of the accelerometer indicates that rework is required, it is relatively difficult to disassemble the accelerometer without damaging one or more of its components.